The present invention relates to pneumatic presses and the like, and in particular to a press controller and method for controlling presses.
Pneumatic presses such as those disclosed in U.S. Pat. Nos. 3,599,561; 3,478,678; 3,545,368; and 3,450,037 are generally well-known in the art, and comprise mating platens which reciprocate through a stroke defined between open and closed platen positions. An air actuated cylinder or ram is positioned generally between the platens, and a valve is positioned between a source of pressurized air and the ram. The valve has an open position in which pressurized air communicates with the power side of the ram to converge the platens to the closed position, and a closed position in which the pressurized air does not communicate with the power side of the ram. A mechanism, such as return springs or the like, are provided to automatically diverge the platens to the normally open position when the valve is closed.
Heretofore, control mechanisms for pneumatic presses have generally comprised a microswitch arrangement. In one such arrangement, similar to that disclosed in U.S. Pat. No. 3,081,657, a microswitch with a pivoting on-off control arm is mounted on the stationary platen, and an adjustable switch stop is mounted on the reciprocating platen. The microswitch is connected to and controls a solenoid operated poppet valve, which opens and closes to reciprocate the air actuated ram and the press platens. During the power stroke of the press, the adjustable switch stop physically abuts the microswitch control arm, and shifts the microswitch into the "off" position to close the valve. The power stroke is thereby halted, and the platens are returned to their normally diverged position. As the press retracts, the control arm is permitted to return to its initial "on" position to begin another stroke cycle of the press.
A major drawback associated with such microswitch press controllers is that they have a relatively slow reaction time. Hence, although certain types of press operations and applications do not require a long stroke of the press, prior controllers have been unable to cycle the press at speeds that provide maximum efficiency and productivity. For example, in one known pneumatic press having a microswitch type of controller, the minimum time that the solenoid valve can remain actuated is approximately 40 milliseconds. Hence, the press speed is automatically limited in accordance with the microswitch lag time, even though a much shorter valve actuation time would accomplish the desired press operation.
Another drawback associated with such microswitch press controllers is that they cause the press to "bounce." Since prior microswitch controllers have a minimum time that the solenoid valve can remain actuated, the minimum length of the press power stroke is also set. When the minimum power stroke of the press exceeds the maximum stroke of the die, the reciprocating platen hits or bottoms out against mating platen stops on the press, causing the press to "bounce." This "bouncing" results in substantial noise, premature press and die fatigue, vibration of the shop floor and adjacent machinery, and can even have unfavorable safety ramifications under certain circumstances.
Hence, it is desirable to provide a controller that is capable of operating the press at maximum speed for a particular operation and application so as to increase production and efficiency, and which consistently regulates the press stroke at a distance that avoids press bouncing.